I'm using Twenty Twelve with one of my sites on WordPress and someone told me that the site wasn't appearing as it should. She told me that instead of the menu, she saw a "MENU" button, which if clicked, gives a long list of all the pages. After some investigating I found that if a browser is resized very small, or when viewing on mobile browsers, the menu is replaced by this button. I'm not sure how big her browser window was, but seeing as she was viewing it on a PC I'd not have thought it was that small.
The funny part is, even when You are on iPhone5 with full HD resolution You still get the MENU button.
My question though, is it possible to disable this feature? It's highly annoying, it really needs to be optional. Having to click 'menu' to get a list of pages just isn't user friendly.